Initial Access (IA) is the process of powering on a wireless device such as a User Equipment device (UE) in order for it to access the cellular network. There are three steps in this procedure, which are fairly independent of which Radio Access Technology (RAT) is being used (the below is inspired by Long Term Evolution (LTE)):                1. Cell search—acquiring network symbol and frequency synchronization (sync) to the network and obtaining fundamental cell information, e.g., the cell Identity (ID), for cell selection.        2. Receiving system information—receiving further cell and network information defining cell and network properties, e.g., operator, carrier bandwidth, system frame number, access information, and adjacent cell information.        3. Random access procedure—this is the step where the UE signals its presence to the network in order for the network to be able to page or schedule it.        
In order to transmit and receive signals at a specific carrier frequency, a transceiver (both base station and device) needs to translate a baseband signal to/from the carrier frequency. This is done by mixing a signal with a local version of the carrier frequency generated in the local oscillator (LO). A LO, in turn, derives its output signal from a crystal oscillator (XO) from which a signal with a fundamental frequency is up-converted or modulated to the desired carrier frequency. The open loop (i.e., prior to the LO having locked to the carrier frequency) relative frequency inaccuracy in a crystal is typically 10-50 parts per million (ppm) depending on the XO frequency and quality. Typically, an XO with higher resonance frequency is needed for higher carrier frequencies in order to cope with the phase noise. However, the higher reference frequency for the XO results in higher relative inaccuracy. This implies that higher New Radio (NR) carrier frequencies will face a fivefold relative frequency inaccuracy compared to LTE at 2-3 gigahertz (GHz). Note that NR is a term used to refer to Third Generation Partnership Project (3GPP) Fifth Generation (5G) NR.
LTE comprises two synchronization (sync) signals, the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS), that are used in order to establish symbol and frequency sync and to obtain, e.g., cell identity (ID). The PSS is used in order to get an initial frequency lock (±4 kilohertz (kHz)) which is further refined in the SSS.
In order to identify prospect sync frequencies, the UE may, in some prior art solutions, perform a frequency scan over the complete frequency band, as illustrated in FIG. 1. From the frequency scan, the UE may obtain the individual frequency carriers from a matched filtering operation in which the LTE spectrum shape is used, typically one shape for each LTE bandwidth, as is illustrated in FIG. 2. FIG. 2 illustrates results of a matched filtering operation for the frequency scan of FIG. 1. This gives the UE an initial understanding of what carrier bandwidths are present in the frequency band, where, and, consequently, at what frequencies to search for PSS and SSS since their positions are fixed to the center of the system bandwidth, as is illustrated in FIG. 3. FIG. 3 illustrates frequency locations to be searched for synchronization signals based on the results of the matched filer operation of FIG. 2. Notably, FIG. 3 is a simplification in that multiple frequencies are typically tested around each peak.
The above identified cell search positions are quite inaccurate though. Furthermore, a simple spectrum analysis does not take into account the possible frequency error that may be present in the UE. Hence, for each identified position, and possibly also adjacent alternative frequencies, there is a need to manage the large initial frequency errors that may be expected at power on, typically up to ±30 kHz at 2.6 GHz. This is done by a grid search in which different frequency error hypotheses are tested in order to identify the most likely one, i.e., the frequency error for which the likelihood of an existing PSS is maximized. Having done that, the UE may continue its cell search procedure by receiving the SSS.
The ambition of the Fifth Generation (5G) cellular system, or New Radio (NR), is to be a one stop shop for connectivity. Hence, a wide and sometimes contradictory requirements specification has been defined by 3GPP. Some of these requirements, and their effect on cell search is described below:                Lean carrier, implying a minimal signaling overhead, in particular regarding broadcast transmissions such as sync signals and broadcast channels. With respect to sync, this may be directly translated to a large sync periodicity.        Flexible band utilization, implying narrow specialized, local networks, e.g., for factory connectivity, should be possible to combine with general purpose, wide area, mobile broadband networks. One parameter in a flexible band utilization is a narrow sync frequency raster such that there are many locations to position sync, in turn providing large network configuration flexibility.        Low sync complexity, such that an initial cell search does not perform worse compared to today's Fourth Generation (4G) LTE networks. Complexity is typically proportional to the sync period and inversely proportional to the sync frequency raster. Dense raster and dense periodicity amount to similar complexity as sparse raster and sparse periodicity.        